There are 2 formulas to determine the mid-point progression or mid-point differential.
The first formula expresses the differentials between adjacent mid-points in dollar value:
Midpoint of the Next Grade = (Midpoint of Lower Grade X Midpoint Differential) + Midpoint of Lower Grade
Midpoint of Lower Grade = $30,000
Midpoint Differential = 10%
Midpoint of the Next Grade = ($30,000 X 10%) add $30,000 = $33,000
The second formula expresses the differentials between adjacent mid-points as a percentage or a ratio.
The formula for calculating it as a ratio is as follows:
Midpoint of the Next Grade less Midpoint of Lower Grade / Midpoint of Lower Grade
That is (Mid B – Mid A) / Mid A
Mid-point of the Next Grade = $33,000
Mid-point of Lower Grade = $30,000
Mid-point Differential = ($33,000 less $30,000) / $30,000 = 0.1
This is 10%